When a defect manifests itself in the wall of a natural gas or oil transmission pipeline, it is the common practice to cut out a section of pipe, having the defect in it, and to weld in a substitute length of new pipe.
Before welding can occur, it is necessary to seal off each of the open ends of the pipeline, to prevent migration of hydrocarbon gases into the welding area. In the past, plugging has been accomplished by stuffing a charge of aqua gel into the open end of the pipeline. After the pipe section substitution has been made and the line has been placed back into service, the fluid being transmitted gradually removes the gel particles without damage to the line equipment.
However, the gel plugs can slump and gas may leak through to the weld area. This occurrence has led to some serious accidents. In addition, the gel is viewed as an unwanted contamination if a high quality fluid, such as jet fuel, is being shipped.
It is therefore desirable to develop a reliable expandable/contractable machanical vapor plug for temporarily sealing the bore of a pipeline. However, the problem to be solved is how to collapse the expanded plug for removal, once the substitute length of line has been welded into place and one cannot physically get at the plug.